Characterization of a novel calcium response element in the glucagon gene

To maintain blood glucose levels within narrow limits, the synthesis and se cretion of pancreatic islet hormones is controlled by a variety of extracel lular signals. Depolarization-induced calcium influx into islet cells has b een shown to stimulate glucagon gene transcription through the transcriptio n factor cAMP response element-binding protein that binds to the glucagon c AMP response element. By transient transfection of glucagon-reporter fusion genes into islet cell lines, this study identified a second calcium respon se element in the glucagon gene (G2 element, from -165 to -200), Membrane d epolarization was found to induce the binding of a nuclear complex with NFA Tp-like immunoreactivity to the G2 element, Consistent with nuclear translo cation, a comigrating complex was found in cytosolic extracts of unstimulat ed cells, and the induction of nuclear protein binding was blocked by inhib ition of calcineurin phosphatase activity by FK506, A mutational analysis o f G2 function and nuclear protein binding as well as the effect of FK506 in dicate that calcium responsiveness is conferred to the G2 element by NFATp, functionally interacting with HNF-3 beta binding to a closely associated s ite. Transcription factors of the NFAT family are known to cooperate with A P-1 proteins in T cells for calcium-dependent activation of cytokine genes. This study shows a novel pairing of NFATp with the cell lineage-specific t ranscription factor HNF-3 beta in islet cells to form a novel calcium respo nse element in the glucagon gene.